There's a seller on Amazon Marketplace selling that D-Link new for $5.99
+ $5.49 shipping = $11.48 total.
5-port switches are typically the smallest you can get. 1 port is used for the "uplink" back to the router, so you can attach up to 4 devices to it. Also, the switches mentioned so far are 10/100 Mbsp "Fast Ethernet" devices. There's even faster 10/100/1000 "Gigabit Ethernet" switches, but they cost more and may not be needed. If you're connecting primarily to the internet, and not connecting between computers and other devices on the network, 10/100 should be fine.
Finally, you don't mention how big the building is, but another option would be to configure another router as a wireless access point (AP) on the other side of the wall. This would extend the wireless network on that side of the wall, and you could use the extra ports on the configured router/AP like a switch.